The present invention relates to the use of aluminum silicate or mixtures of aluminum silicate and silica to form low thermal conductivity insulation bodies.
Over the years, considerable research and development work has been focused on the design and development of more effective thermal insulation. These efforts resulted in the invention and development of fibrous insulation, for example, fiberglass and mineral wool. Fibrous insulation systems have thermal conductivities that range from 0.230 to 0.350 BTU-IN/HR-SQFT-F. Further work resulted in the development of gas-expanded polymeric materials: foamed polystyrene and polyurethane. These materials had conductivities as low as 0.210 BTU-IN/HR-SQFT-F. The introduction of high molecular weight, chlorofluorocarbon (CFC), gas into polyurethane resins resulted in foams with extremely low thermal conductivity: 0.120 BTU-IN/HR-SQFT-F.
Over 100 years ago it was discovered than an evacuated space provides an effective barrier to the transfer of heat. In fact, it is actually more effective than any modern day insulation. At that time however, and for many years thereafter, its use was confined to the insulation of Dewar flasks, or as they are commonly known, vacuum bottles. This vacuum insulation technology was restricted to this application because only the cylindrical geometry of the flask would prevent collapse of the wall, when the hollow space between the walls was evacuated.
For this unique insulation technology to be applied more widely to other insulation applications, it was necessary to develop evacuated, flat panels. To this end, development work has focused on the filler material to be used in the space between the walls. The selection of this material is critical in making a low conductivity panel since when it is compressed by atmospheric pressure, the filler often becomes very conductive.
In recent years, many investigators have developed and patented evacuated, flat panel insulation. One of the earliest of these patents is U.S. Pat. No. 3,179,549 (Strong et al). It describes an evacuated, flat panel insulation that consists of fiberglass enclosed in a metal container. This type insulation panel was very costly to make and had the added disadvantage of high thermal conductivity caused by heat transfer around the edges of the panel by the metal container. U.S. Pat. No. 4,159,359 (Pelloux-Gervais et al) describes the use of fumed silica as the filler material in an evacuated, flat insulation. The cost of a panel wherein fumed silica is used as the filler material would be prohibitive. Barito et al (U.S. Pat. No. 4,636,415) describes the use of precipitated silica as a filler material in an evacuated, flat panel insulation. Although less costly than fumed silica, precipitated silica is far more costly than aluminum silicate of the present invention. U.S. Pat. No. 4,681,788 (Barito et al) describes the use of mixtures of precipitated silica and fly ash. These compositions are stated to have thermal insulating properties equivalent to evacuated panels made with precipitated silica alone. The major advantage to these mixtures is cost; fly ash, being the residue (ash) of burned coal, is a very inexpensive material. Blending this less costly material with the more costly precipitated silica, reduces the overall cost of the finished panel. The major drawback to the use of fly ash is the inconsistancy of the material. A result of this inconsistency are variations in the thermal performance of panels made with fly ash. On the other hand, aluminum silicate and silica of the present invention are manufactured products whose properties are stringently controlled.